A number of important criteria must be met when designing a radio frequency antenna for mobile use. These criteria fall within two major categories namely electrical and mechanical. Of the electrical characteristics, Frequency Bandwidth, Voltage Standing Wave Ratio (VSWR) at various frequencies, power gain of tranmission and reception, impedance match at various frequencies, polarisation, receive/radiation pattern, radiation and radiant power transmission and loss characteristics, tunability and repeatability are considered important. Of the desirable mechanical characteristics size, weight, robustness, minimization of vibration induced noise sources, vibration rating, weather resistance and configuration of radiating and reflector elements are considered important.
These criteria assume increased or decreased importance dependent on the frequency bandwidth and ground plane conditions required of the transceiver and antenna system.
This invention relates particularly to those antennas which are required to operate in portions of the electromagnetic frequency spectrum which require an optimum antenna length that cannot be physically supported as in the circumstances of a mobile transceiver and its vehicle mounted antenna.
An antenna has an equivalent circuit configuration comprising a capacitance and resistance in series which physically comprises a wire or metallic body commonly referred to as the radiating element. For a vehicle mounted antenna the radiating element is usually orientated vertically, however, for operation in the High Frequency band, the antenna must be physically of lesser length than the optimum quarter wave length, and therefore requires inductance.
The inductance element resonates with the antenna inductance and capacitance at the operating frequency to provide a resistive impedance for the transceiver. As the antenna is required to operate over a wide frequency range this inductance must be variable to achieve resonance over the required range.
The most robust arrangement is the use of a fixed inductance, which however only provides a narrow frequency bandwidth of optimum operation. Also well known is the use of variable inductance which ideally provides a broader frequency bandwidth.
Commonly used to provide a variable inductance means, is an arrangement of coils used singly or in combination (ganged). However, the physical size of these coils and their enclosure prohibits adjacency to the antenna and requires manual or automatic adjustment of the coil combinations to achieve tuning for specific frequencies. Their large size also creates an installation problem, as space is usually at a premium in mobile conditions and thus frequency bandwidth is ultimately restricted.
Thus it is also known to position an inductive coil or coils of suitable dimensions and wire size onto the top, intermediate or bottom of an antenna. These coils are mounted coaxial with the antenna's radiating element and if a fixed inductance is used, they may, dependent on their configuration, provide high gain for a narrow frequency bandwidth or lower gain for a broader frequency bandwidth while concommitantly affecting the receive/radiation pattern in conjunction with the voltage and current distribution. The impedance of the antenna also depends upon the diameter and configuration of wire conductor in relation to wavelength. If the diameter of the conductor is increased, the capacitance per unit length increases and the inductance per unit length decreases. Since the radiation resistance is little affected, the decreased inductance/capacitance ratio causes the Q of the antenna to decrease, so that the resonance curve becomes less sharp and hence the antenna is capable of operating over a wide frequency range. The Q of an equivalent circuit is given by the following: EQU Q=wL/R.sub.s
where
w=2.pi. times frequency; PA1 L=inductance; and PA1 R.sub.s =the effective resistance of the circuit.
If variable inductance means are employed, the electrical characteristics of frequency bandwidth are kept as broad as possible, the V.S.W.R. is minimized for each of the infinite frequencies achievable within the bandwidth provided, the gain of the antenna is as high as possible across the bandwidth provided, Q is kept as high as possible, impedance is matched across the frequency bandwidth, the radiation pattern of the commonly vertical polarization receiving/radiating element is omnidirectional in the horizontal plane and achieves the broadest coverage of the azimuthal plane, allows the highest possible level of radio frequency energy to be transmitted and ensures timely, accurate and repeatable tuning of the antenna to the desired frequency.
In addition to the above ideal electrical characteristics, the antenna must also exhibit manageable mechanical attributes such as a size which allows mounting on to the mobile platform, a weight as low as possible to minimize both the inertia of the antenna, and the required strength of the mounting means. An antenna employing variable inductance means for tuning must also be particularly robust in its mechanical working since any movement of contacts will directly affect the tuning of the antenna.
In the prior art, variable inductance means has comprised a coil positioned at the base of a whip antenna element, having a wiper contact that is adjustable either manually or automatically along the external length of the coil, which also exhibits the following problems and fails to meet ideal design criteria for a mobile antenna.
In fact, there exist a number of electrical and mechanical problems which arise as a consequence of the use of this configuration. Commonly the large degree of both high and low frequency vibration which is experienced in all planes of movement impart both a strain on the top of the coil and wiper contact resulting in physical departure of the contact from the coil or along the coil. This has the effect in the first instance of adding noise to both received and transmitted signals while in the second instance changes the tuned frequency from that desired by the transceiver. The bulky nature of the external wiper adds significantly to the turning moment of the mounted antenna and adds additional weight and size to the antenna configuration.
Some prior art antennae exhibit spurious resonance which is not a consequence of the transmitter output but rather a resultant product of the combination of unused coil, capacitance and stray radio frequency energy. This spurious resonance is also present during receive periods and both occurrences can affect antenna gain and transceiver performance in particular by decreasing the Q of the antenna and heating up the unused inductance.
It is a further characteristic of prior art antennae that the movement of the external wiper becomes very critical within specific zones of the coil and stable frequency tuning relies on the constant positioning of the wiper on the coil to an extent greater that can be provided.
External wiper arrangements require mechanically complex movement mechanisms and additionally require them to be protected and shielded from mechanical damage.
Therefore, it is an object of this invention to provide an antenna having a means for varying inductance for tuning an antenna which receives and transmits radio frequencies and which overcomes the abovementioned problems associated with the prior art.
It is an aspect of the invention to provide an antenna having characteristics which meet or exceed prior art antenna performance specifications which employs variable inductance means for tuning.
A further aspect of the invention is to provide an antenna having a coil with a moveable wiper positionable along the coil which simplifies the construction and operation of such an antenna.
A still further aspect of the invention comprises an antenna according to the above description wherein the method of manufacturing the tuning coil further comprises the use of a mandrel, wherein a pair of tubes of insulating material are located on each end of the mandrel whereupon an intermediate portion of the mandrel is left exposed, and a coil of electrical conducting material is wound onto the intermediate portion of the mandrel, so as to form a tuning coil, whereupon a chemically setting resinous material is placed over the external surface of the coil and a further tube of insulating material is then positioned over the external surface of the tuning coil. The further tube extends over a portion of each of the first mentioned tubes, such that the resinous material adheres the further tube both to the coil and the first said tubes. Once the resinous material has set, the mandrel can be removed, and in order to ease the removal of the mandrel, a mould release agent can be applied to the mandrel prior to winding of the coil. Once the mandrel has been removed, the internal surface of the tube can be honed so as to remove any surplus resinous material or insulating material surrounding the coil, thereby exposing a conducting surface of the wire coil.
Yet a further aspect of the invention comprises an antenna tuning coil manufactured according to the above description wherein the coil of electrical conducting material is wire wound in a single helical configuration or in combination with a plurality of helically wound wires of different diameter in parallel and contiguous with each other and joined at the beginning and end of the helically wound wire. This arrangement can maintain the broadband characteristics of the antenna while keeping the Q high.